configure: Allow capstone=git only if git update is not disabled

Even with --disable-git-update, ./configure tries updating the capstone
submodule instead of marking it "no"; this disables capstone submodule
if git update is disabled.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
Reviewed-by: Thomas Huth <thuth@redhat.com>

configure

@@ -4568,7 +4568,7 @@ case "$capstone" in
   "" | yes)
     if $pkg_config capstone; then
       capstone=system
-    elif test -e "${source_path}/.git" ; then
+    elif test -e "${source_path}/.git" -a $git_update = 'yes' ; then
       capstone=git
     elif test -e "${source_path}/capstone/Makefile" ; then
       capstone=internal